The US military caught London and other Nato allies by surprise when it rushed into halting on-the-ground operations with Afghan security forces, after four Americans and two British soldiers were killed by local policemen at the weekend.

Military sources said the suspension of routine joint operations, with approval now required from a general for Nato and Afghan soldiers to serve together, had been under discussion for several weeks following a surge of "insider attacks". Fifty-one Nato soldiers have been killed by members of the Afghan security forces, some of them by Taliban infiltrators, in 36 attacks this year – 15 of them last month alone.

But the Pentagon called a halt out of concern at the effect on army morale, and damage to public and political opinion over attacks from within the same military that Nato is training.

Allies were only informed after the order had been written, causing a scramble to downplay its significance by the International Security Assistance Force (Isaf) command and limit the political fallout in London and other Nato capitals.

Opinion is divided on what the final impact will be on military operations, given that the joint on-the-ground operations that saw Nato and Afghan troops patrolling and fighting alongside each other were central to the US-led strategy to train the 350,000-strong Afghan army to hold off the Taliban after the American pullout in 2014.

"There's certainly been a fraying of the trust. There's no doubt about that," said lieutenant general David Barno, the commander of coalition military operations in Afghanistan for two years from 2003, and now retired from the army.

"One of the fundamental questions is: is this a temporary injunction to provide Nato and Isaf time to come up with some new measures that ensures the safety of their troops that are out on an advisory roles, partnering roles? Or is this going to be a longer-term restriction that prevents Nato troops serving in these very important roles? I think the jury's out on that."

Barno said the suspension of joint operations is a message from the US that the insider attacks are "a very serious problem" and "this is probably an appropriate response to it at this juncture".

Stephanie Sanok, a Pentagon official in the Bush administration with experience of Afghanistan and Iraq, who is now with the Centre for Strategic and International Studies in Washington, said insider attacks are "a real problem and challenge facing the coalition" and that stopping joint operations while Nato "figures out exactly who they're sharing bases with is long overdue".

But she said it is a blow to the strategy to build a capable Afghan army to confront the Taliban.

"From an operational perspective it does hinder the coalition's ability to hand over responsibility to capable partners. It hinders in terms of confidence levels in the Afghan national security forces, because they're not going to be partnering with them as much. When you partner less, obviously their capabilities are going to be less," she said.

Lieutenant colonel Daniel Davis – who caused a political stir in Washington in February by accusing the Pentagon of "lying" about the situation in Afghanistan because his experience during a year-long deployment "bore no resemblance to rosy official statements by US military leaders about conditions on the ground" – said that calling off of joint operations will be damaging because it will reinforce a perception among Afghans that the US is rushing to leave.

Davis said "insider attacks" have eroded trust among Nato troops of their Afghan colleagues. But, he added, confidence between the two militaries has been on the wane for some time because of overly optimistic claims by the US about the state of the war with the Taliban and Barack Obama's setting of a 2014 date for an end to American combat operations.

"In my personal opinion, we (Isaf) have been responsible for a portion of the destruction of trust between the Afghan forces and Isaf troopers because so often our leaders say things like "everything's on track", "we're on the right azimuth."

"But when those messages are heard by the Afghan government, the Afghan security forces, and even the Taliban, they see with their own eyes that nothing could be further from the truth. When they hear us saying these things and actually appear to believe them, they either don't trust us or they don't put any value in our ability to assess," Davis said.

"When you're using the language of success to describe abject failure, you have no credibility in the eyes of those on the ground who know the truth."

Davis said the pullback from joint operations will be taken by many Afghan soldiers as further evidence that the US will abandon them.

"Now a lot of them are getting scared because as we're getting closer to the date in 2014 when we're completely gone, they know they're going to be on their own, unable to operate without us. Despite the constant claims that we're not abandoning Afghanistan again, they see that we are in fact leaving many of them to their own fate," he said. "I fear that as time continues to pass between now and 2014 the level of trust may degrade further. I can see where they might think: not only are you going to abandon us in 2014, you're starting to cut the cords now."

Sanok agreed with Davis that setting a deadline for withdrawal has caused the Afghan soldiers to question US commitment.

"I understand why people like to set deadlines and have aspirations. But it really is the conditions on the ground and they really are so changeable in Afghanistan," she said. "I think the vast majority of them will roll their eyes and say this [the halt to joint operations] is just another example of them abandoning us, and why should we care?"

Sanok said some in the US military command view the suspension of joint operations as an opportunity.

"The Afghan national security forces are going to have to take over responsibility anyway. Part of the US military establishment is saying: now or 2015, it doesn't matter. The Afghan national security forces are either going to sink or swim. They're just going to have to swim sooner rather than later," she said.

Sanok does not agree with that view, and said she regards continued partnering as essential to getting the Afghan army into shape. But she is sceptical that Afghan soldiers are motivated to fight the Taliban.

"It's not just capability, it's willingness. We can give them as much training and equipment and advice as possible. But if they're not willing to shoulder that burden, it won't work," she said.

That, said Davis, is the fundamental flaw in the US approach.

"I conducted several patrols with both US and Afghan troops. They [the Afghan soldiers] weren't actually interested in learning anything. They only did what they were made to do. The rest of the time, they're lying in the shade somewhere, resting. In the environment where our word means little to them, they don't seem interested in learning anything," he said.

Barno is not so pessimistic about the Afghan army, but he said that the "insider attacks" are a warning as to the determination of the enemy it faces.

"One of the things it clearly says to me is: this is a resilient and adaptable enemy who is still very dangerous and very capable. The Taliban is going to find seams and fissures and fractures inside of the coalition's capabilities to exploit such as insider attacks, which have immense political repercussions back in Nato countries," he said.

"They will exploit that, and they will leverage that to the maximum extent. So this is a very crafty, very wily enemy and he is not at all on ropes or crushed at this point in time. He's simply adapting to the situation he finds himself in."